Fuel cells can offer potentially clean, quiet and efficient power generation. Unlike thermal energy based engines, fuel cells use an electrochemical or battery-like process to convert the chemical energy associated with the conversion of hydrogen gas (and carbon monoxide for high temperature fuel cells) into water (and carbon dioxide for high temperature fuel cells) into electricity. Among various types of fuel cells, solid oxide fuel cells (SOFC) use hard ceramic compounds of metal oxides (e.g., calcium or zirconium oxides) to form their components, for example, electrodes, electrolytes and interconnects. Typically, in solid oxide fuel cells, oxygen gas (O2) is reduced to oxygen ions (O2−) at the cathode, and a fuel gas, such as hydrogen (H2) or a hydrocarbon, such as methane (CH4), is oxidized with the oxygen ions to form water and carbon dioxide (from hydrocarbon) at the anode. If a hydrocarbon is used as the fuel gas, then carbon dioxide (CO2) is also produced and becomes part of the exhaust from the anode of SOFC (anode exhaust). The anode exhaust typically includes about 15% to about 30% unreacted fuel gas. Despite the advantages of clean and quiet power generation, fuel cell systems have faced a number of formidable market entry issues resulting from product immaturity, over-engineered system complexity, fuel efficiency, etc. Fuel efficiency can be increased by employing larger surface areas of the anode and cathode, or by increasing the number of fuel cells in a fuel cell stack. However, these approaches typically result in increases in the size of the fuel cell stack. It is a considerable challenge for an SOFC stack to achieve high fuel utilization efficiency due to the limitation of cell voltage and uniform fuel distribution.
Therefore, there is a need for developing methods of increasing fuel efficiency in fuel cell systems, and for developing fuel cell systems having high fuel efficiency, and in particular fuel cell systems of relatively small size.